Particular types of electro-optical modulators have been recently developed, such as, for example, described in U.S. Pat. Nos. 3,958,862 and 4,125,318. The operation of these modulators depends on the effect of applying a voltage to a symmetrical electrode pattern to induce periodic change of the refractive index in an electro-optical element in the region of the surface of the element where the light is totally internally reflected. The resulting symmetrical periodic phase change induced in the wave-front of a light beam directed through or at the electro-optical element produces a far field pattern of the Raman-Nath form exhibiting symmetry about the zero energy order. The zero order of the output beam can then be modulated by adjusting the applied voltage to the electrode pattern to alter the distribution of the light from the zero order into higher orders, and as often desired, to eliminate the zero order.
The electro-optic effect, in general, permits extremely rapid and direct modulation of a light phase front with an electronic drive signal without creation of a sound field as an intermediate step and modulation of the phase fronts with negligible loss of incident light energy, permitting high light throughput efficiency. Unfortunately, previous electro-optic device configurations have been quite complex to fabricate, sensitive to thermal fluctuations, and required high drive voltages and powers. Low drive voltages and powers are feasible with waveguide configurations wherein the light energy is confined in a small volume by total internal reflection, but these devices have not yet demonstrated commercial impact.
An exception to this is the TIR modulator configuration described in the aforementioned patents. This device is easy to fabricate, rugged, relatively insensitive to temperature drift and thermal loading, and extremely fast. It requires a low drive voltage (10 volts to 80 volts is typical) and has a low input capacitance (20 pf. to 80 pf is typical).
The unique characteristics of the TIR modulator which make it attractive for a number of applications are:
(1) High electric fields, and hence high light phase front deformation are achieved with low drive voltages by placing the drive electrodes close together. Electrode pitches of 100.mu. are typical, but pitches of 1.mu. may be achieved.
(2) Because the electric field induced by the electrodes is very closely confined to the electrode surface, the interdigitated structure has low capacitance. This is an extremely valuable characteristic for high frequency drive.
(3) The phase grating structure, as distinct from the Pockel's cell modulator configuration, is inherently rugged and temperature-insensitive.
(4) No light beam confinement is necessary, other than simple focusing.
(5) Even though the electric field induced by the electrodes is a skin field, with shallow penetration into the electro-optic crystal, adequate interaction of the light beam with the electric field is assured by a grazing reflection off the electrode surface.
A line composer, for the purposes of this invention, is defined as a system which converts an electronic signal pattern into an image consisting of a line of light, with a light intensity profile along the line specified by the electronic signal pattern. A line composer can be used to image a single point of light on the image line at a specified line position, and to change this line position in time in a prescribed manner.
An example of a practical line composer is the light valve used in the Scophony television system of the 1930's (see for example, D. M. Robinson, "The Supersonic Light Control and Its Application to Television with Special Reference to the Scophony Television Receiver", Proc. IRE, Vol. 27, August 1939, pps. 483-486). The video signal modulates the light phase front through the photo acoustic effect using as an intermediate mechanism a modulated sound carrier moving through a liquid cell. The sound carrier defines a spatial carrier frequency, i.e., effectively a phase diffraction grating which breaks up the light incident upon the cell into discrete diffraction orders. The phase front modulation is converted into a corresponding intensity modulation at the image line through the techniques of phase microscopy.
A drawback to the Scophony light valve is that the sound field moves with the speed of sound through the liquid cell. The image created by such a light valve must also move, because the image is a direct mapping of the sound carrier profile. To freeze the image, a rotating mirror is required in the optics train. A more serious drawback to the Scophony light valve is a resolution and speed limitation imposed by useful light valve apertures and by sound absorption-i.e., materials limitations.
In TIR configurations to date, all the electrodes are joined into two blocks of electrodes, a single drive potential being applied to these two blocks. A periodic light phase front deformation with constant magnitude over the electrode area results. What would be desired is to provide a line composer which utilizes a TIR device, having the structural advantages set forth hereinabove, to convert periodic light phase front deformations generated by an electronic signal pattern into an image consisting of a line of light with a light intensity profile specified by the pattern, such a system providing, for example, a simplified, economical replacement for the rotating polygonal members presently being utilized in raster scanning systems.